The activity and subcellular localization of Ca2+/calmodulin(CaM)-dependent protein kinase II (CaMKII) are regulated by Ca2+/CaM binding, autophosphorylation at Thr286 and Thr305/306, and interactions with CaMKII- associated proteins (CaMKAPs). Inhibition of CaMKII, mutation of CaMKII autophosphorylation sites, or blocking CaMKII binding to NMDA receptor (NMDAR) GluN2B (formerly NR2B) subunits disrupts normal LTP at hippocampal glutamatergic synapses. However, at other excitatory synapses CaMKII is important for LTD induction or for synaptic changes independent of kinase activity. The specific molecular processes engaged by CaMKII to induce these disparate changes of synaptic function in different physiological situations remain unclear. The overarching goal of this project is to define mechanisms of CaMKII action toward specific downstream targets that are critical for normal regulation of excitatory synaptic transmission. Our unifying hypothesis is that CaMKII actions are micro-regulated in discrete subcellular compartments by changing the composition of CaMKII complexes with its substrates and other proteins. Thus, the repertoire of downstream CaMKII actions in a specific cellular/physiological context will be dictated by other components of these complexes. In previous funding periods, we made excellent progress characterizing CaMKII complexes containing NMDAR subunits, 21/22 subunits of voltage-gated Ca2+ channels, densin, 1-actinin and/or SAP97, generating 20 primary research publications. Interestingly, these CaMKAPs interact with CaMKII by multiple molecular mechanisms, and also typically interact with other synaptic proteins. Our findings suggest novel mechanisms for precise modulation of CaMKII-dependent actions on key synaptic signaling proteins, as well as for regulated assembly of multi-protein complexes in postsynaptic densities (PSDs). This competing renewal application proposes the use of a combination of biochemical, molecular and electrophysiological approaches in vitro, in heterologous cells, and in neurons/brain slices to test the following specific hypotheses: Aim 1: Test the hypothesis that 2 subunits and densin differentially target CaMKII isoforms to regulate L-type voltage-gated Ca2+ channels. Aim 2: Test the hypothesis that 1-actinin activates CaMKII to regulate GluN2B- NMDARs and that phospholipids modulate this pathway. Aim 3: Test the hypothesis that CaMKII interacts with and phosphorylates specific SAP97 splice variants to regulate AMPARs. Aim 4: Test the hypothesis that CaMKII autophosphorylation is modified by CaMKAPs within PSDs, and in turn regulates relevant protein- protein interactions, allowing CaMKII to play a non-catalytic, structural role. These studies will define fundamental mechanisms of physiological control that allow CaMKII to selectively regulate targets critical to different forms of synaptic plasticity, learning and memory.